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quency of 2450 MHz. The oven was provided with five different power setlings designated as high, medium high, medium, medium Iow and Iow. Weight measurements were madę with a gibertini Model E415 electronic balance with sensitiyity up toO 001 g.

A simple method to determine tha power Icvel oł microwaveoven ishy usingthewater load method (Mac Larchy 1980). In this method, a known mass m of water with specific heat c is heated for a given period At and the average temperaturę rise of the mass, AT is measured. The average microwave power absorption P is given by

_{P =} ^mcAT    (i)

At

In the measuring of thisquantity, certain para-meters musi be taken into account, such as size, heat capacity and electrical properties of the Container, position of the Container in the oven, duty cycle of the inagnetron source and cooling effects during the mea-surements. The average power delivered to the load calculated from equation( 1) is also dependent on the volume of the load. This is due to the reflection of the microwave radiation at the surface of the load.

In order to standardize this measuremenl, Copson (1975) recommended that the average power output of the microwave oven be calculated using 1000 ml of distilled water placed in a pyrex beaker and heated for 120 seconds. Applying this recommendation, Table 1 shows the observed average microwave power of fivc difTerent setlings. These settings are at 100%, 70.7%, 55.8%, 32.2% and 13.9% of the maximum power of 743 ± 1 watts.

Following the procedurę of the standard method as described in the introduction, the

TABLE 1

Temperaturę rise for 1 litre of distilled water and heated for 120 seconds at various setting of microwave oven. ( orrespondingoutput power is calculated using the water load method.

Indication	Temperaturę rise	( C) Output power (± IW)
High	21.3	743(100%)
Med high	15.05	525 (70.7%)
Med	11.88	415(55.8%)
Med Low	6.86	239 (32.2%)
Iow	2.95	103(13.9%)

sheeted coagulum was placed in a petri dish and dried in ihe microwave oven. The sample was taken

TABLE 2

Optimum drying time of sheeted coagulum at varioi power levels with 10 g initial mass of latex.

Output power (W)	Finał drying time (Min)
743 (High)	6.5
525 (Med. High)	8.5
414 (Med.)	12.3
239 (Med. Iow)	16.0
103 (Low)	29.0

out and weighed every 30 seconds, until the lossi mass between successive weighings was less than mg. In the above DRC determination, the initi moisture contenl of the sheeted coagulum befor drying was about 20 to 40%.

RESULTS AND D1CUSSION

Fig. 1 shows the microwave drying curvesofsheetc coagulum for different power Ievels with all initi mass of latex of 10 g. It is evident from this figur that at the highest power level, the finał drying tiir takes less than 10 minutes and the average dryin ratę is about 0.2 g/min. The overall drying rai inereases with inereasing power levd. The efTect morę pronounced at the lower power level with a average drying ratę of about 0.05 g/min. Th dryingcurvesshow rapid drying ratę at the beginnin of the drying time as the moisture from the sampl is removed. The absorbed microwavc power decreased and the drying charactcristics approac those of convective dryi-ng. The microwave pow level versus finał drying time (for 10 g initial mai of sample) is shown in Fig. 2 and the data for ił optimum drying time are shown in Table 2.

Fig. 3shows the dry ing curve for various initi. masses of latex ranging from 5 - 30 g with an aven ge microwave power of about 734 watts. Therc not much difference in finał drying time for diffe entvaluesof initial mass of sheeted coagulum. Th property due to the absorbing power of the sampl depends on the amount of moisture in the sheeti stated earlier. This is confirmed in Fig. 3 where tł initial drying ratę inereases with inereasing amoufll of sample.

The results show that the optimum drying lin for sheeted coagulum is much longer than fre* latex as in the TSC determination. The different in the heating ratę of the fresh latex and sheetf coagulum depend on the dielectric properties < the two. At 2.45 GHz (microwave oven frequeno the dielectric loss inereases with the increasir

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PERTANIKA VOL. 14 NO. 1,1991